Cast iron alloy

ABSTRACT

The invention proposes a cast iron alloy for cast iron products which are highly thermally stable, the alloy containing, as nonferrous constituents, at least the elements C, Si, Mo, Al and, as admixtures, Ni, Mg and/or S, and the C content being less than 2.9% by weight. The alloy is used, for example, to produce exhaust manifolds or turbocharger casings.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a cast iron alloy for cast iron productswith a high thermal stability, the alloy containing, as nonferrousconstituents, at least the elements C, Si, Mo, Al and, as admixtures,Ni, Mg and/or S.

[0002] In the automotive industry, cast iron alloys are used to producecastings which have to be highly thermally stable, for example the partswhich come into contact with the hot exhaust gases from the internalcombustion engine. Since internal combustion engines are producing everhigher powers, the exhaust-gas temperatures are becoming ever higher.The exhaust manifolds and casings of turbochargers are exposed totemperatures of 900 to over 1000° C. The transformation temperature,i.e. the temperature at which one type of crystal in the alloy istransformed into another type of crystal, plays an important role atthese high temperatures. At the transformation temperature, the volumechanges and this volume change leads to irregular expansioncharacteristics on the part of the castings. The alloys which can beused must have transformation temperatures which are higher than themaximum temperature of use. Austenitic cast steel alloys or alloys witha high nickel content are also used for the abovementioned high usetemperatures. Nickel is a relatively expensive raw material. The exhaustmanifolds are often also formed from sheet-metal parts, in which casethe poor sound insulation makes its presence adversely felt. Thesesolutions are relatively complex to produce.

[0003] U.S. Pat. No. 5,236,660 discloses a cast iron alloy of thegeneric type. The alloy contains approximately 3.1% by weight of C, 4.6%by weight of Si, 1.9% by weight of Al, 1% by weight of Mo and ifappropriate also admixtures of Co and Nb.

[0004] Working on the basis of this prior art, it is an object of theinvention to provide a cast iron alloy which is made from elements whichare as inexpensive as possible, the castings having as high a usetemperature as possible.

SUMMARY OF THE INVENTION

[0005] The foregoing object is achieved by providing a cast iron alloyfor cast iron products having a high thermal stability, wherein thealloy comprises containing, as nonferrous constituents, at least theelements C, Si, Mo, Al and, as admixtures, Ni, Mg and/or S, and the Ccontent being less than 2.9% by weight and preferably 2.5 to 2.8% byweight.

DETAILED DESCRIPTION

[0006] In accordance with the present invention, the alloy should havethe highest possible structure stability and be as resistant as possibleto oxidation. The foregoing is achieved by providing a Si content ofbetween 4.7 to 5.2% by weight and an Al content of between 0.5 to 0.9%by weight. Molybdenum is added in an amount of between 0.5 to 0.9% byweight.

[0007] The invention provides a cast iron alloy which has atransformation temperature of more than 950° C., has no disruptiveexpansion properties when used in conjunction with internal combustionengines and which can be produced as inexpensively as possible in acasting process. The graphite in the cast iron alloy may be spheroidalor vermicular. The amount of nickel added remains restricted to 0.1 to1% by weight. Magnesium and silicon may be present in trace amounts ofup to 0.05% by weight. It has proven particularly advantageous to addzirconium in the range from 0.1 to 0.4% by weight immediately before itis cast into its final shape. Zirconium has a favorable influence on theoxidation resistance and the mechanical strength. The zirconium may beadded in the form of a prealloy.

EXAMPLE 1

[0008] An exhaust manifold for an internal combustion engine of apassenger car made from nodular cast iron with the following chemicalcomposition in percent by weight: 2.6 C, 5.1 Si, 0.1 Ni, 0.6 Mo, 0.6 Al,0.6 Zr, 0.04 Mg and less than 0.01 S has a ferritic microstructure.Measurements carried out in a dilatometer result in a longitudinallinear expansion coefficient of 16.10⁻¹⁶/K, which makes it possible toconclude that the transformation temperature is over 950° C. In a hottensile test at a temperature of 300° C., the following mechanicalstrength values were determined: Rp_(0.2)=575 N/mm², Rm=600 N/mm² andA=0.4%.

EXAMPLE 2

[0009] An exhaust manifold for an internal combustion engine of apassenger car made from cast iron with vermicular graphite having thefollowing chemical composition in percent by weight: 2.6 C, 5.1 Si, 1.0Ni, 0.7 Mo, 0.6 Al, 0.3 Zr, 0.02 Mg and 0.02 S has a ferriticmicrostructure. Measurements carried out in a dilatometer result in alongitudinal linear expansion coefficient of 16.10⁻¹⁶/K, which makes itpossible to conclude that the transformation temperature is over 950° C.In a hot tensile test at a temperature of 300° C., the followingmechanical strength values were determined: Rm=545 N/mm² and A=0.1%.

[0010] If the temperature does not drop below 1460° C. during castingand if the elements Al and Zr are only added immediately prior tocasting, for example as an Al—Zr prealloy, it is possible to produceferritic cast iron products with a transformation temperature of over950° C. The products produced in this way are distinguished by a verylow change in volume as a function of the temperature, a good thermalshock resistance, good mechanical properties, a good resistance tooxidation and a low raw material price.

[0011] This invention may be embodied in other forms or carried out inother ways without departing from the spirit or essentialcharacteristics thereof. The present embodiment is therefore to beconsidered as in all respects illustrative and not restrictive, thescope of the invention being indicated by the appended claims, and allchanges which come within the meaning and range of equivalency areintended to be embraced therein.

1. A cast iron alloy for a cast iron product characterized by highthermal stability, the alloy comprising, as nonferrous constituents,positive additions of C, Si, Mo, Al, wherein C is present in an amountof less than 2.9% by weight.
 2. The cast iron alloy as claimed in claim1, wherein the C content is 2.5 to 2.8% by weight.
 3. The cast ironalloy as claimed in claim 1, wherein the Si content is 4.7 to 5.2% byweight.
 4. The cast iron alloy as claimed in claim 1, wherein the Mocontent is 0.5 to 0.9% by weight.
 5. The cast iron alloy as claimed inclaim 1, wherein the Al content is 0.5 to 0.9% by weight.
 6. The castiron alloy as claimed in claim 1, wherein the alloy further comprisesNi, wherein the Ni content is 0.1 to 1.0% by weight.
 7. The cast ironalloy as claimed in claim 1, wherein the alloy further comprises Zr,wherein the Zr content is 0.1 to 0.4% by weight.
 8. The cast iron alloyas claimed in claim 1, wherein the graphite fraction is spheroidalgraphite.
 9. The cast iron alloy as claimed in claim 1, wherein thegraphite fraction is vermicular graphite.
 10. A cast iron alloycomprising between 2.5 to 2.8 wt. % C, 4.7 to 5.2 wt. % Si, 0.1 to 1.0wt. % Ni, 0.5 to 0.9 wt.% Mo, 0.5 to 0.9 wt.% Al, 0.1 to 0.4 wt. % Zr,Mg and S each up to 0.05 wt. % max, and balance essentially Fe.
 11. Thecast iron alloy as claimed in claim 1, wherein the cast iron productcomes into contact with exhaust gas from an internal combustion engine.12. The cast iron alloy as claimed in claim 1, wherein the cast ironproduct is an exhaust manifold for receiving exhaust gases from aninternal combustion engine.
 13. A process for producing the cast ironalloy as claimed in claim 7, wherein the Al and Zr are added as an Al—Zrprealloy immediately before the alloy melt is cast.
 14. A process forproducing the cast iron alloy as set forth in claim 13, wherein thetemperature of the alloy melt is over 1460° C. immediately prior tocasting.
 15. In combination, an internal combustion engine and a castiron product, the cast into product being contacted with exhaust gasesfrom the internal combustion engine, the cast iron product comprises acast iron alloy comprising, as nonferrous constituents, positiveadditions of C, Si, Mo, Al, wherein C is present in an amount of lessthan 2.9% by weight.